Outlets such as grills or ceiling diffusers are used in the air distribution systems of an air conditioning installation in order to control the direction and velocity, and consequent cooling or heating effect, of the outlet air. These devices generally comprise some form of divergent outlet which is connected to ducting of an air conditioning system supplying heating or cooling air to a room. In order to provide optimum air distribution in the room, flow deflection means such as vanes are provided at the divergent outlet.
A ceiling diffuser is generally used in the case of ceiling distribution of conditioned air. Such a diffuser may be round, square or linear in shape. When cooling is called for, an optimum cooling air distribution is achieved by projecting the cool primary air from the diffuser laterally across the ceiling. This minimises waste in cooling the warmer air in the region above occupants of the room, by providing cooling over the ceiling and walls and occupied floor region only. However, since the primary air is cooler and therefore more dense than the secondary room air it will tend to drop to the floor if lateral projection is not sufficient, so that ideal distribution and mixing with room air may not be achieved and draughts may result. The performance of the diffuser from a cooling point of view thus depends on the amount of lateral spread which can be achieved by the diffuser.
When heating is called for, optimum air distribution is achieved by projecting the heated primary air down to the floor. However, since the primary air is warmer and therefore less dense than the secondary room air, there is a tendency for the warmer air to stay at ceiling level. This lessens the effect of mixing of the primary air with the room air, resulting in a colder floor region with floor to ceiling stratification and wasteful heating of the ceiling region. The performance of the diffuser from a heating point of view thus depends on the ability of the diffuser to direct air downwards. Hence as well as ensuring comfort of the occupants, optimum air distribution is desirable to minimise heating or cooling loads by heating or cooling only those regions of the room where it is required.
To meet the dual requirements of a diffuser to be able to operate efficiently in both heating and cooling situations, with seasonal changes, variable geometry ceiling diffusers have been developed wherein the flow pattern may be varied depending upon requirements. Variation in geometry is generally achieved either manually or by electrically or pneumatically operated mechanisms which are controlled by a thermostat so as to adjust the diffuser configuration.
Disadvantages associated with systems involving electrically or pneumatically operated mechanisms are that they require external electrical or pneumatic connections, and are therefore invariably complicated and/or require periodic maintenance. Hence these systems are generally only economical where the savings in heating/cooling loads can justify the additional costs.
Systems involving manually operated mechanisms are generally used for lower cost installations. In these systems the diffuser mechanism is set once or twice a year at the time of change between heating and cooling demands. However as well as involving labour cost these systems are not able to cater for unseasonable changes in heating/cooling requirements, resulting in incorrect diffuser settings, causing discomfort and excessive heating/cooling loads.
As a lower cost alternative to the above variable geometry diffusers, a compromise may be achieved by having a fixed geometry diffuser which provides both a lateral spread and a downward flow pattern. Although the cost of this type of diffuser may be lower with no maintenance requirements, heating/cooling loads are inevitably higher due to the non-optimum room air distribution, and ideal comfort conditions may not therefore be achieved.